(1) Field of the Invention
The invention relates generally to electronic oscillators and in particular to improved crystal resonator, thin-film resonator or micro electromechanical resonator oscillators, realized with monolithic integrated-circuit technologies, where one chip solutions include automatic gain control to accommodate a highly accurate frequency-generation exhibiting low phase noise and stable amplitudes at higher frequencies.
(2) Description of the Prior Art
Most crystal oscillators in monolithic integrated circuit technology are developed using Pierce oscillator circuit schemes, where the frequency determining resonator is working in parallel resonance mode. Realized with quartz crystals as resonators normally only narrow band tuning is featured and phase noise is considered sufficiently good at frequency offsets not too far away from the oscillator carrier signal. It would be advantageous to extend the tuning range whilst maintaining a good phase noise behaviour at far away offsets.
Crystal-controlled oscillators have been in use for decades in electronic systems as frequency references; but such oscillators have mostly been implemented using bi-polar transistors as active elements. However, the dominant technology for the fabrication of most integrated circuits today is CMOS and design techniques for highly stable crystal oscillators in this technology are less well known, especially when it comes to frequencies of about 100 MHz, as necessary for modem communication applications.
In the prior art, there are different technical approaches for achieving the goals of good tuneability and low phase noise. These crystal oscillator arrangements always include a piezo-electric, e.g. quartz, crystal and drive current means therefore. Unfortunately, these approaches are somewhat expensive, both in terms of technical complexity (e.g. differential push-pull or balanced bridge structures, extra filter or tank circuits, sophisticated temperature compensation or gain control circuits, amplitude peak detectors etc.) and hence commercial costs. It would be advantageous to reduce both expenses. This is achieved by using an oscillator circuit working with a crystal in serial resonance mode, originating from Butler. Using the intrinsic advantages of that solutionxe2x80x94as described later on in every detailxe2x80x94the circuit of the invention is realized with standard CMOS technology at low cost.
Several prior art inventions describe related crystal oscillators.
U.S. Patent (U.S. Pat. No. 3,996,530 to Feistel et al.) describes a Butler oscillator with an amplitude limiting amplifier followed by a filter network interposed between the voltage amplification stage and the impedance matching stage. This allows both the voltage amplification stage and the impedance matching stage to be operated in a linear mode at all times which assures that a piezoelectric crystal connected between the stages is connected in a relatively low, constant impedance path and is driven by a sinusoidal waveform, free of distortion, to assure maximum frequency stability.
The circuit of the invention is realized using bipolar technology.
U.S. Patent (U.S. Pat. No. 6,052,036 to Enstrom et al.) discloses a highly stable single chip crystal controlled oscillator with automatic gain control and on-chip tuning. An amplitude detector monitors the output of a crystal controlled oscillator amplifier and produces a feedback signal proportional to the output signal of the amplifier to ensure oscillation is induced at startup and that the amplitude of oscillation is limited to a preselected value during operation to conserve power consumption by the amplifier. The capacitor tank circuit connected to the input of the amplifier includes a voltage variable capacitor the voltage across which is initially established at manufacture to tune the oscillation frequency to a preselected value. The voltage across the voltage variable capacitor is also adjusted to compensate for temperature variations in the circuit.
U.S. Pat. No. 6,194,973 to Williamson) shows an oscillator with automatic gain control, where an oscillator having an adjustable gain circuit provides abundant gain when the oscillator is first powered up but reduces the gain substantially below its start-up value once oscillations build up, thereby substantially reducing the power consumed. The oscillator comprises an inverting amplifier coupled to a resonator, an oscillation detector coupled to the inverting amplifier amplifier, and a common-gate amplifier coupled to the oscillation detector. The inverting amplifier amplifies oscillations of the resonator according to a gain. The oscillation detector outputs a detection signal in response to oscillations of the resonator. The level of the detection signal is proportional to the amplitude of the oscillations. The common-gate amplifier receives the detection signal and, in response, limits the current to the inverting amplifier to control the gain based on the level of the detection signal.
U.S. Patent (U.S. Pat. No. 6,259,333 to Shimono) describes a temperature compensated quartz oscillator. A system that provides an accurate frequency generating source, avoids mode coupling of the quartz vibrator, and has a high production efficiency. A high frequency amplifying circuit uses a bridge circuit as part of a feedback circuit, and a quartz vibrator is inserted in a branch side connecting the CR-circuit. The oscillation frequency is less than the serial resonance frequency of the quartz vibrator.
U.S. Patent (U.S. Pat. No. 6,278,338 to Jansson) discloses a crystal oscillator with peak detector amplitude control. A crystal oscillator apparatus is described that has a wide dynamic frequency range and that is capable of supporting a broad range of crystal types. The present invention reduces the unwanted side effects that are associated with the prior art crystal oscillator designs, such as the clipping of signals, the introduction of signal distortion and unwanted signal harmonics. The present invention reduces the total wasted loop gain of the oscillator while also reducing the amount of integrated circuit real estate required to implement the crystal oscillator. The crystal oscillator apparatus of the present invention preferably comprises a crystal resonator circuit, an inverting amplifier, a bias circuit, a reference circuit, and a peak detector circuit. The present invention takes advantage of automatic gain control design techniques. The gain of the present crystal oscillator is automatically regulated using a closed loop circuit design. The present invention advantageously utilizes a peak detector circuit in combination with a reference circuit. The peak detector compares a reference signal with an amplified and inverted oscillation signal produced by a crystal resonator, and generates a feedback signal as a result of the comparison. The feedback signal controls a bias circuit that, in turn, controls the amplified inverted oscillation signal.
In that invention cited here, the crystal is being operated in a parallel resonance Pierce oscillator configuration.
In the paper from Vittoz et al.xe2x80x94cited here as [Vittoz, et al., xe2x80x9cCMOS Analog Integrated Circuits Based on Weak Inversion Operationxe2x80x9d, IEEE Journal of Solid-State Circuits, vol. SC-12, No. 3, June 1977. pp. 224-231. ]xe2x80x94an automatic gain control circuit is described, the disadvantage of the tuned small band operation of this circuit however is avoided in the current invention.
A principal object of the present invention is to provide an effective and very manufacturable method and circuit for generating resonator stabilized oscillation signals. The results are especially applicable and very efficient for use with resonators at least made up of Quartz or Piezo crystals, or of Thin-Film Resonators (TFR) or Micro Electro Mechanical System (MEMS) resonators but not restricted to only those types of resonators.
A further object of the present invention is to attain a low amplitude distortion of the oscillator signal.
Another still further object of the present invention is to reach a low phase noise behaviour of the circuit, i.e. to substantially enhance its frequency stability.
Also an object of this invention is the starting up of the generation process of the oscillation signal at maximum speed.
Another object of this invention is minimizing the distortion and the phase noise of the generated oscillation signal in steady state operation.
A still further object of the present invention is to reduce the power consumption of the oscillator circuit by realizing inherent appropriate design features.
Another further object of the present invention is to reduce the cost of manufacturing by implementing the circuit as a monolithic integrated circuit in low cost CMOS technology.
Another still further object of the present invention is to reduce cost by minimizing the chip area by means of effectively minimizing capacitance values.
In accordance with the objects of this invention, a circuit, capable of generating a stable, resonator controlled oscillation signal is achieved. Said circuit comprises means for generating an oscillating signal having a stable and predetermined oscillation frequency with means for driving said oscillation generating means and for matching the impedance levels between said oscillation generating means and this driving means. Also comprised are means for amplifying and controlling said generated oscillation signal of said oscillation generating means. The circuit further comprises means for connecting said amplifying and controlling means to said driving means in a closed loop feedback fashion. Still further comprised are means, coupled between said driving means and said amplifying means for controlling the amplification factor (gain) of said amplifying means in such a way, that a steady oscillation signal is maintained and means for isolating said amplified oscillation signal of said amplifying means from influences resulting from changing output loads at said isolating means. Finally are comprised means for generating the bias currents, based upon the supply voltage level, for said driving means and for said isolating means.
Still in accordance with the objects of this invention, a method for generating a stable, amplitude controlled oscillation signal with an electronic device or technology is given. Said method includes providing a resonator element for determining of the oscillator frequency and providing a White cascode buffer circuit for driving this resonator element. The method further comprises providing a common gate amplifier circuit for amplifying the oscillation signal together with providing an automatic gain control circuit for stabilizing the oscillation signal. Also included in the method is providing an output buffer circuit for isolating the oscillator circuit from load influences and providing a bias block circuit for delivering the biasing reference currents for the buffer circuits. The method is equally essential for controlling the closed loop generation of the oscillations.